1. Field of the Invention
This invention pertains to hoisting winches of the type designed to lift and lower heavy loads, and more particularly, to self-energizing braking and drive systems for such winches.
2. Description of the Prior Art
Braking and planetary drive systems for hoisting winches are of the type, such that when the winch is driven in the hoisting or "up" direction the drum is rotated to lift the load. The power is generally provided by a hydraulic motor. When the motor is stopped a brake is automatically set to prevent the load from turning the drum. One procedure for allowing this one-way hoisting direction of the drum is through the use of an over-running clutch, that is, a clutch which allows rotation in one direction but does not allow rotation in the opposite direction. In hoisting winches it is known to overcome the brake when powering out a load in the load-lowering condition. In one technique this is done by running the hydraulic motor in the opposite direction and using hydraulic pressure to overcome springs on the braking discs to allow the winch drum to be rotated in the lowering condition. In these winches if the load, because of changes in static and dynamic friction, begins to accelerate and cause the drum to run faster than the hydraulic motor, then the hydraulic motor begins to act as a pump reducing its inlet pressure and thus reducing the pressure acting against the springs on the braking discs. This action automatically sets the brake to reduce the speed of the drum. There are problems using the hydraulic fluid as the means for overcoming the brakes. One of these problems, for example, is that the brake becomes sensitive to hydraulic back pressures in the hydraulic power system as well as sensitive to any other type of desirable pressure increases on the input side of the hydraulic pump.